1. Field of the Invention
This invention relates generally to a multiple quantum well infrared detector, and more particularly, to a multiple quantum well infrared detector incorporating a series of tightly coupled well groups.
2. Discussion of the Related Art
Infrared detectors have a wide range of applications typically for the detection of heat gradients in devices such as infrared security systems and thermal imaging systems. Known types of detectors include mercury cadmium telluride (HgCdTe) detectors and silicon extrinsic detectors for the detection of infrared photons. An HgCdTe infrared detector includes a P-I-N semiconductor which operates on the principle of detecting infrared photons by measuring electrons which are released from the valence band to the conduction band in the intrinsic layer from the absorption of the infrared photon energy. A silicon semiconductor detector also detects infrared radiation by absorption of infrared photons in which bound electrons in the impurity energy bands of the dopant atoms gain the absorbed photon energy. The electrons which absorb the photon energy are released into the conduction band enabling an electric field bias to direct the released electrons to specific terminal contacts where they can be measured. Both of these types of infrared detectors suffer the drawback that they cannot be effectively integrated with certain readout circuits incorporating field effect transistors (FET) which enable the detected photons to be imaged.
A multiple quantum well (MQW) infrared detector is known in the art which is able to integrate with the FET readout circuitry not compatible with the HgCdTe and silicon detectors discussed above. The conventional MQW is a single well device which incorporates an array of barrier and well layers, typically aluminum gallium arsenide (AlGaAs) and appropriately doped gallium arsenide (GaAs), in an alternating pattern forming a multiple of single well structures. A dopant electron bound in the well structure formed by the conduction band of the GaAs and AlGaAs must acquire enough energy from an absorbed infrared photon to reach the conduction band of the AlGaAs in order to be a free carrier measurable as induced current by infrared photon radiation. This single well type structure only provides a single allowable energy state within the well structure which can be occupied by an electron. This paucity of allowable energy states effects the likelihood that an electron will absorb enough photon energy to be released into the conduction band.
The conventional MQW, therefore, still suffers a number of drawbacks making it less efficient as a high performance infrared imaging device. Specifically, the single well MQW has a limited band width range of the infrared spectrum which it can detect. Further, the conventional MQW is substantially sensitive to the angle of incidence of the infrared photons. Consequently, the prior art MQW is limited as a high performance detector compatible with the circuitry of many readout devices, such as thermal imagers.
What is needed then is an infrared detector which is adaptable to readout circuitry, such as will be found in a thermal imager, and has a higher band width detection and is less sensitive to angle of incidence than its prior art counterpart. It is therefore an object of the present invention to provide such a detector.